The instant invention relates to vehicle passenger restraint systems and, more specifically, to a firing circuit for an air bag passive restraint system which can be readily diagnosed for the presence of faults therein.
Known air bag passenger restraint systems comprise a firing circuit having a voltage supply providing a potential across a firing squib in series with a first and second normally open vehicle acceleration sensor, each of which is shunted by a resistor of like nominal resistance. A small current thus flows through the circuit while the sensors remain in the normally open condition. The closure of the sensors upon collision or marked deceleration of the vehicle generates a significant rise in the current flowing through the squib which, in turn, fires the squib to deploy the air bag. In order to accommodate both driver and passenger air bags, a separate firing circuit is employed for each air bag, each firing independently during a vehicle collision.
An advantage of such separate firing circuits is that the presence of a fault therein, i.e., a component failure such as the improper closure of one of the normally open sensors, is readily diagnosable by measuring the voltage potentials about the circuit. Unfortunately, since each squib is connected to only two sensors, there are no redundant fire paths for the squibs. Thus all four sensors must close in order to deploy both bags. Redundant fire paths can be created by adding additional dedicated series firing circuits for each air bag. Each air bag would thus have two or more squibs which deploy the bag upon closure of both sensors dedicated to one of the squibs, with the number of redundant fire paths therefore being equal to the number of redundant series firing circuits. Unfortunately, the resultant increase in the number of circuit elements dramatically increases both the cost of a passenger restraint system employing such circuits and the likelihood of failure of such circuit elements.
Another known air bag firing circuit offers multiple firing paths by placing in series a first pair of normally open sensors wired in parallel, a pair of squibs wired in parallel, and a second pair of normally open sensors, also wired in parallel. The sensors are again shunted by resistors of like nominal resistance. The squibs fire upon simultaneous closure of either of the first pair of sensors and either of the second pair of sensors. The provision of multiple firing paths thus increases system reliability without the need for increasing the number of circuit elements used therein. Unfortunately, such circuits are not readily diagnosable--although the examination of voltage potentials about the circuit would indicate that a fault had occurred, there is no way to identify the precise fault, short of removing and testing each circuit element individually until the source of the fault is revealed. For example, upon the failing closed of one of the sensors comprising the first pair of sensors, both sensors must be removed and individually tested to determine which of the sensors had failed.